Seawater is the salty water that fills Earth's oceans and seas. Unlike the fresh water you drink from a tap or find in most lakes and rivers, seawater contains dissolved salt and minerals. If you've ever accidentally swallowed a mouthful while swimming at the beach, you know that distinctive salty taste.

The salt in seawater comes from minerals that rivers wash from rocks and soil over millions of years, carrying them downstream to the ocean. As ocean water evaporates into clouds, the salt stays behind, making the ocean saltier over time. On average, seawater is about 3.5% salt, which means roughly one cup of salt in every gallon of ocean water.

This saltiness makes seawater undrinkable for humans and most land animals, even though it covers more than 70% of our planet's surface. Ships that spend months at sea must carry fresh water or use special machines called desalination plants to remove the salt. The salt content also makes seawater denser than fresh water, which is why it's easier to float in the ocean than in a swimming pool. Marine animals like fish, whales, and dolphins have special adaptations that let them thrive in seawater, processing or filtering out the salt their bodies don't need.